This disclosure relates generally to the field of digital image capture operations. More particularly, but not by way of limitation, it relates to techniques for focus score improvement by noise cancellation in a digital camera.
A camera's autofocus system automatically adjusts the camera lens' position to obtain focus on a subject. As used in this disclosure, the term “camera” refers to any device having digital image capture capability. Examples include, but are not limited to, digital SLR cameras, point-and-shoot digital cameras, mobile phones, laptop or notebook computer systems, tablet computer systems, personal digital assistants, and portable music/video players.
Autofocus systems may generally be divided into two types: active, and passive. Active autofocus systems measure the distance to a subject by emitting, and using, a signal to estimate the distance to the subject (e.g., ultrasound and infrared). The estimated distance is then used to adjust or set the camera's focal length (i.e. lens position). In contrast, passive autofocus systems set a camera's focal length or lens position by analyzing an image captured by the camera's optical system. Passive autofocusing can be achieved through phase detection or contrast measurement.
Many small multifunction devices such as mobile phones use a passive autofocus technique based on contrast measurement. In devices such as these, autofocus operations involve adjusting the position of the device's lens at a specified number of locations and evaluating the focus (e.g., contrast) between corresponding areas in successive images. That lens position is given a focus score, corresponding to the maximum contrast, as determined by the number of sharp edges detected. The focus score informs the autofocus system if the lens position is the position that has the maximum sharpness.
However, a problem many autofocus systems suffer from is that they may trigger, or fail to trigger, an autofocus operation improperly based on the amount of noise present within the focus score. When this happens, the autofocus system may improperly determine that a camera is out of focus or in focus, and improperly adjust (or fail to adjust) the position of the camera's lens.